We have found that, in contrast to young women and young elderly men, older women do not adapt to endurance exercise training with an improvement in maximal cardiac output. It is our hypothesis that this is due to a deficiency of sex hormones. We propose to study the independent and combined effects of endurance exercise training and estrogen and progesterone replacement therapy (HRT) on left ventricular size and function and on arterial stiffness in 60-72 yr old women. These studies will test the hypothesis that 1) HRT in combination with endurance exercise training will induce left ventricular volume-overload hypertrophy, and improve left ventricular contractile function, 2) HRT in combination with endurance exercise training will increase total blood volume, 3) HRT alone will decrease arterial stiffness, and this effect will be enhanced by endurance exercise training, 4) HRT plus endurance exercise training will improve left ventricular diastolic filling dynamics, and 5) the above changes will be reflected in a significant augmentation in maximal cardiac output. Sixty to 72 yr old subjects will be randomly assigned to one of 4 groups; 1) exercise alone, 2) HRT alone, 3) exercise plus HRT, and 4) control group. The exercise program will consist of a 2 month flexibility program followed by 9 months of endurance exercise training. The HRT will consist of cyclic conjugated equine estrogen (Premarin 0.623 mg) from day 1-25, and medroxyprogesterone acetate (Prover 5 mg) from day 16 to 25 each month for 11 months. Maximal cardiac output will be assessed by two-dimensional echocardiography and Doppler studies under basal conditions and during Beta- and alpha- adrenergic stimulation induced by isoproterenol and phenylephrine respectively. Blood volume will be measured by determining red cell and plasma volumes by standard radionuclide techniques. Arterial stiffness will be evaluated by measurement of pulse wave velocity.